Dense Media Separation (DMS)—also known as Heavy Media Separation—is a process widely used in the mining industry to separate the valuable minerals from the non-valuable rock by differences in density. For example, DMS can be used in the diamond industry because diamond is denser than the host rock, and also in the iron ore industry because haematite is denser than silica. In the coal industry where coal is less dense than silica, DMS may also be used.
The DMS process involves the use of a suspension of particulate material in a liquid, typically water. The particulate material, or media, preferably comprises magnetic particles, for example magnetite or ferrosilicon (FeSi) particles because this facilitates the recovery of the particulate material for reuse after the separation process. The particles of the particulate material are sufficiently fine to allow their stable suspension in the relevant liquid, and typically take the form of powder, while being sufficiently dense/heavy to provide the required media density. For example, a 350 mm cyclone operating in the diamond industry on +1 mm to 4 mm kimberlite using ferrosilicon as the suspension media uses medium with approximately 90% of the media particles finer than 44 micrometers. The media particles are typically formed by milling or atomisation. The resulting media suspension is commonly referred to as a dense medium. Where the particulate material comprises magnetic or magnetised particles, the media suspension may be referred to as a magnetic dense medium. The media suspension has a density greater than that of the liquid alone. For example a typical dense medium may have an apparent density of, say, 2.65 specific gravity while the specific gravity of water is 1. The advantage of using a magnetic particulate material is to facilitate subsequent retrieval of the particulate material for reuse.
During use, the media suspension is contained in a separation vessel, for example a cyclone vessel (sometimes referred to as a dense medium cyclone). The media suspension is usually mixed with the solids to be separated (typically comprising ore but is also used in the recycling industry for metal and plastic recycling) before being transferred to the separation vessel. Where the separation vessel comprises a cyclone, separation is effected by differences in centrifugal force experienced by particles of the solids to be separated of differing density, the less dense material tending to float in the liquid suspension and so exiting the cyclone at the top, while the denser material sinks and exits through the bottom.
A problem with DMS is that the suspended media tends to separate from the media suspension along with the solids to be separated as a result of its relatively high density (typically between 6.7 and 7.1 specific gravity for ferrosilicon). Therefore a stable media is required for optimum DMS efficiencies, and optimum efficiencies are a priority more than ever with high commodity prices. Stability is achieved using powdered media that is fine enough to prevent rapid settling of the media under the centrifugal forces in the cyclone or gravity in the case of a Dense Media Drum. It is this fineness that gives rise to most media losses for reasons including the following:                1. Fine suspension media adheres to the ore/solids surface and is difficult to wash off from the recovered product at the end of the process. This is a particular problem for porous materials, such as coal.        2. Fine suspension media is more susceptible to corrosion (e.g. oxidation) due to the high surface area to volume.        3. Fine suspension media is more difficult to recover in magnetic separators. The higher hydrodynamic drag forces that fine particles experience, results in poor recovery of finer media in the magnetic separators.        
Commercially available Ferrosilicon is manufactured as either milled or atomised. The atomised version is commonly manufactured in five size fractions: Special Coarse, Coarse, Fine, Cyclone 60 and Cyclone 40 and, because it is spherical, it is more easily washed, more resistant to corrosion but is more expensive. Milled ferrosilicon is cheaper and is commercially available in six different sizes: 100#, 65D, 100D, 150D, 270D, 270F (from for example DMS Powders (www.dmspowders.com) or M & M Alloys Limited (www.mandmalloys.com). In conventional DMS plants where the required media SPECIFIC GRAVITY is greater than 3.2, as in iron ores, the viscosity of the milled media is too great for efficient separation and atomised ferrosilicon is used.
Generally, the smaller the cyclone diameter, the larger the centrifugal forces experienced by the media particles in the cyclone and finer media is required for good stability. Larger cyclones have lower centrifugal forces and the media particles do not need to be so fine for stability. However, feed pressure of the combined solids and media suspension is usually increased with increasing cyclone diameter and coal DMS plants operating with magnetite as the media tend to use one particle size for all cyclone diameters.
Typically, ferrosilicon losses in cyclone DMS circuits range from 120 g ferrosilicon per tonne (g/t) up to 500 g/t. Magnetite is a cheaper alternative to ferrosilicon. However, magnetite is less dense than ferrosilicon and therefore losses tend to be higher. Media losses are known to represent from 20% to 40% of the total operating costs of a DMS plant.
It would be desirable to reduce media losses in DMS systems.